Thank you for your question. I have just a few questions to help me with yours.
Is she painful at all?
Is she weak in both back legs even thought the muscle is decreased in just the one?
Does she ever stumble or trip?
Does she have any signs of illness...such as drinking more water, changes in appetite?
Without examining her I obviously can not say for sure, but this sounds a lot like a degenerative myelopathy. Beagle are predisposed to this condition. In simple terms degenerative myelopathies refer to the nerves in the spine that communicate to the back legs becoming degenerative or not sending signals as well. It is a completely non painful disease. The consequence of this is weakness and incoordination in the rear legs. It does cause muscle deterioration in the rear legs. Usually both rear legs are affected equally. Degenerative myelopathies have no treatment. They do tend to be slowly progressive.
However, this could be any kind of myelopathy (spinal problem) including compression for a disk, some kind of inflammatory problem, or an emboli. I think these are less likely because they tend to involve some pain. You probably should have a vet take a look at her just so you don't miss something that is potentially treatable.
I hope this helps answer your question, but if you have more questions let me know.
Arthritis can cause some of these signs, but usually causes more pain and stiffness that is worst when they have been sleeping, but improves at the warm up and move around. It doesn't usually cause their legs to give out, but can cause reluctance to jump. What you describes fits more with a myelopathy, but it could be a combination of both. I would have a vet look at her because you can do a lot to relieve the discomfort of arthritis with medications.
is there anway i can tell if she has myelopathy?
One thing you can try to do, but definately isn't diagnostic is check how aware she is of her feet. You do this by having her stand evenly on all four feet. Support her a bit with an arm under her belly. Then without changing the position of there leg flip her foot upside down so the top of her foot is touching the ground. She should instantaneously correct the foot. If it takes her 10 seconds or more to realize that her foot is not right....that is very suggestive of a myelopathy.